A light-emitting diode (LED) is principally formed by a multiple epitaxy structure of a light-emitting semiconductor material. For example, a blue LED is chiefly formed by a gallium nitride based (GaN-based) epitaxy film, which is internally a PN structure with one-directional electrical conductivity.
The above-mentioned blue LED usually adopts a sapphire substrate to grow a high-quality GaN-based epitaxy film thereon; however, a sapphire substrate has poor thermal and electrical conductivities. Therefore, conventional blue LED can only adopt a lateral structure having positive and negative electrodes located at a same side of the substrate. Thus, a light-emitting area of the LED element is decreased, and the resistance and forward voltage drop are increased due to a current crowding effect.
Please refer to FIGS. 1A and 1B for the schematic diagrams of a conventional LED element before and removing a sapphire substrate respectively. To improve the foregoing defects, a method used nowadays for manufacturing a high-performance LED element is to first grow a GaN-based epitaxy film 2 on a sapphire substrate 1. A binding layer 3 is then grown on the epitaxy film 2, and a new substrate 4 is formed on the binding layer 3. By laser lift-off or chemical mechanical polishing, the sapphire substrate 1 is further removed to allow the GaN-based epitaxy film 2 to remain on the new substrate 4. The new substrate 4 has a high heat dissipation coefficient and good electricity conductivity and is therefore more adaptable to high driving current applications, thereby solving issues such as heat dissipation under a high lumen flux of an LED element.
However, in the above-mentioned method of removing the sapphire substrate, the laser lift-off process is costly and produces stress which is difficult to control and may cause fragmentation. Moreover, the chemical mechanical polishing process may cause problems such as low polishing accuracy, difficulty in controlling a polishing range, and over-polishing or under-polishing.